My dog has a rash could this be mange?!?

This happened over night. We left her in the yard for a few hours and this morning she had a huge red lumpy rash all over her inner and outer back legs. She's been scratching like crazy. She eats normal dog food and is very well taken care of. I don't know what this could be. There are bees outside could it be that? She has an appointment with the vet this week. But I'm just trying to get an idea of what it could be.